px4io: return raw ADC value for current

we don't know how to scale it as we have no info on what sensor is
attached. As we are returning a uint16_t it is better to let the FMU
sort it out or we'll just lose precision.
px4io: changed adc_measure() to return 0xffff on error, and lower timeout

the timeout of 1ms was far too long, and could impact flight
performance

Returning 0xffff on error matches the FMU code, and allows bad values
to be discarded
px4io: fixed voltage/current output and add discharged_mah calculation

this integrates the current over time to calculate discharged_mah, and
allows the scaling of the current and the bias to be set with the
px4io command
px4io: handle errors from adc_measure()

don't update the voltage/current values on error